1. Field of the invention
The present invention relates to improvements in an internal combustion engine exhaust system, and more particularly to an exhaust gas purifying device for reducing particulate matter.
2. Description of the Prior Art
Various types of exhaust gas purifying devices in a diesel engines have been proposed. Major types of them are provided with traps for trapping particulate matter in order to prevent the particulate matter from exhausting into atmosphere. The trap is periodically regenerated by reburning the particulate matter to avoid the degradation of the engine performance.
One of typical devices is disclosed, for example, in Japanese Utility Model Provisional Publication No. 61-173712, in which the exhaust gas purifying device is provided with a bypass passage which is disposed at the upstream side of a trap. This device is further provided with an oxidation catalytic converter in the bypass passage in order to provide a reaction heat to the trap by means that fuel is supplied upstream of the oxidation catalytic converter and generates the reaction heat with unburnt gases. Since it is difficult that the trap is regenerated at a lower temperature than 400.degree. C., fuel is supplied to the catalytic converter to rise the exhaust gas temperature by the oxidation reaction heat.
However, difficulties have been encountered in the above-discussed system, in which the system presents a condition that the exhaust gas temperature is lower than a regeneration temperature even if the fuel is supplied to the catalytic converter. If such a condition is maintained, particulate matter is further accumulated in the trap and the vehicular performance is largely degraded. Further, the supplied fuel into the exhaust gas is incompletely burnt and transformed into white smoke.